The present invention relates generally to methods of power control for CDMA communication systems and, more particularly, to methods of reverse link power control during soft handoff in a wireless communication system.
Numerous access schemes exist to allow multiple users to share a communication medium. One such access scheme is known as code division multiple access (CDMA). In CDMA systems, multiple users share the same carrier frequency and may transmit simultaneously. Each user has its own pseudo-noise (PN) sequence, which is approximately orthogonal to the PN sequences of other users. Transmissions to or from individual users are imprinted with that user's PN sequence. The receiver selects the desired signal, which combines in the communication with unwanted signals, by correlating the received signal with the PN sequence of the desired signal. All other signals are spread by the PN sequence and appear as noise to the receiver.
The current standard for CDMA systems in the United States is contained in a specification published by the Telecommunications Industry Association and Electronics Industry Association (TIA/EIA), known as IS-95. New standards for wideband CDMA are currently being developed in North America, Europe, and Japan, which offer significant performance improvements compared to the current CDMA standard. One such standard is known as cdma2000. cdma2000 is a wideband, spread-spectrum radio interface that uses CDMA technology to satisfy the needs of third generation wireless communication systems. Several enhancements of the cdma2000 standard have been proposed to facilitate the gradual evolution of third generation wireless communication systems. The cdma2000 variant known as 1xEV-DO is being developed to provide high-speed packet data services as an overlay to existing circuit-switched networks. The next step in the evolution of the cdma2000 technology is the variant known as 1xEV-DV. Networks implementing this standard will provide integrated voice and high-rate data services.
CDMA systems are interference-limited systems. Since all mobile stations operate at the same frequency, internal interference generated within the system plays a critical role in determining system capacity and voice quality. The transmit power from each mobile station must be controlled to limit interference while maintaining desired performance objectives, e.g., bit error rate (BER), frame error rate (FER), capacity, dropped-call rate, coverage, etc.
Two closely related techniques used in CDMA systems to reduce interference are power control and soft handoffs. Power control is used on the reverse link in CDMA systems to control the power of signals received at each base station from the mobile stations. The purpose of power control is to assure that each mobile station served by a particular base station provides approximately the same signal level to the receiver at that sector. In CDMA systems, the system capacity is maximized if the transmit power level of each mobile station is controlled so that its signals arrive at the base station receiver with the minimum required signal-to-noise ratio (SNR) or signal-to-interference ratio (SIR). The target value for the received power level is the minimum level possible that allows the link to meet the predetermined performance objectives.
As the mobile station moves around within the network, the channel conditions change continuously due to fast and slow fading, shadowing, number of users, external interference, and other factors. Power control algorithms dynamically control the transmitted power on the reverse link to maintain the minimum SNR or SIR at the base station under all conditions. Both open loop and closed loop power control are used on the reverse link. In open loop control, the mobile station monitors the received signal strength on the forward link and varies its transmit power inversely in proportion to the measured signal strength. Fading sources in mobile radio systems require much faster power control than is possible with open loop control. Fast power control is provided by the closed loop power control mechanism. In closed loop power control, the base station measures the strength of the received signal from the mobile station and sends power control commands to the mobile station requesting the mobile station to either increase or decrease its transmit power. The power control commands typically comprise power control bits (PCBs), which are sent at a rate of 800 bps. A bit value of “1” commands the mobile station to decrease its transmit power. A bit value of “0” commands the mobile station to increase its transmit power.
Another technique used in CDMA communication systems to reduce interference is known as a soft handoff. A handoff is the act of transferring support for a mobile station from one sector to another when the mobile station moves between cells. In a traditional “hard” handoff, the connection to the current base station is broken and a connection is made with the new base station to resume communication with the mobile station. This is known as a “break before make” handoff. Because all sectors in a CDMA system use the same frequency, it is possible to make the connection to the new base station before terminating the connection with the current base station. This is known as a “make before break” or “soft” handoff. A soft handoff requires less power, which reduces interference and increases system capacity.
During soft handoff, each base station participating in the handoff receives transmissions from the mobile station over its assigned code channel. The code channel assignments are independent and, in general, will be different in each cell. The mobile station transmit power is controlled by the base station that is receiving the best signal, so that the minimum necessary power is transmitted by the mobile station. This is a key requirement if the maximum overall system capacity is to be achieved. Each base station participating in a soft handoff makes a separate determination of the power control bit (PCB) to be sent to the mobile station. The mobile station processes the power control bits from each of the base stations in its active set separately and performs an “or of the downs” logic operation. That is, if any of the base stations transmits a “down” bit, the mobile station reduces its transmit power. The net result is that the transmit power level of the mobile station is reduced to the minimum level needed to be received by the base station with the best reverse link. Thus, the soft handoff mechanism reduces interference in CDMA systems.
In 1xEV systems, the mobile station receives data on the forward link from only one base station at any given time. Instead of combining transmit energy from multiple base stations, the mobile station switches communications from one base station to another. This process is known as sector selection. The mobile station measures the instantaneous carrier-to-interference (C/I) ratio of the pilot signal received from each base station in its active set and requests service from the base station providing the strongest signal.
Another feature of 1xEV systems is the use of line adaptation on the forward link. Transmissions from a base station to the mobile stations are time-multiplexed and transmitted at full power. At any given time, a given base station is transmitting to only one mobile station. The slot times and data rates allocated for transmissions to the mobile stations depend on the channel conditions seen by each mobile station. The mobile stations measure the signal quality on the forward link and send rate control information in the form of either signal quality measurements or data rate requests back to the base station on the reverse link. The base station selects a forward link data rate and assigns slot times for a mobile station based on the rate control information from that mobile station. The base stations vary the modulation and encoding used for the forward link channel, depending on the requested data rate.
The sector selection and data rate requests are transmitted from the mobile station on a reverse link channel that is referred to herein as the reverse rate control channel (R-RCCH). In 1xEV-DO systems, the R-RCCH is known as the data rate control (DRC) channel. In 1xEV-DV systems, the R-RCCH channel is known as the reverse channel quality indicator channel (R-CQICH). In both cases, the R-RCCH conventionally has a fixed power offset relative to the reverse pilot channel (R-PICH). In order to serve a mobile station, the serving base station must be able to receive and reliably decode the R-RCCH.
A mobile station in soft handoff for which the serving base station is different from the best reverse link sector is said to experience “link imbalance.” The factors causing link imbalance are numerous: different forward and reverse link frequencies, different sector transmit power ratings (e.g., when deploying both macro-cells and micro-cells in the same frequency assignment), different transmission losses (e.g., arising from antenna misalignments), and different base station receiver sensitivities (e.g., different reverse link loading or different receiver noise figures). Link imbalances in excess of 3 dB are the norm, and link imbalances in excess of 6 dB are not infrequent in deployed systems.
A mobile station in soft handoff experiencing link imbalance may not be able to receive service on the forward link since the reverse link is power controlled by a base station other than the serving base station on the forward link. The base station receiving the strongest signal will force the mobile station to reduce its transmit power to the level needed to maintain the minimum SNR at that base station. Due to differences in path loss to the serving sector and the reverse link sector, the serving sector may not be able to receive and reliably decode the R-RCCH. If the serving sector is unable to decode the rate control channel, the mobile station cannot be scheduled for service. Indeed, the serving base station may not even be aware that it was selected by the mobile station as the serving base station.